As means for achieving high-precision time (phase) synchronization between base stations, which is necessary in time division duplex (TDD) mobile communication systems, the use of global navigation satellite systems (GNSS) such as GPS (Global Positioning System) is growing. Navigation satellites in global navigation satellite systems carry high-precision clocks that are synchronized to Coordinated Universal Time (UTC), and radio-transmit navigation satellite signals that are synchronized therewith, and it is possible to synchronize the time to UTC by receiving these navigation satellite signals at any geographical point on earth.
Because there is a propagation delay until the navigation satellite signals (hereinafter referred to as satellite signals) from a satellite reach the reception point, in order to correct for the delay time, satellite signals from at least four satellites must be received simultaneously to identify four parameters (x, y, z, t) including the three-dimensional coordinate information (x, y, z) for the reception position of the satellite signals, and reception time information (t). In the case of GPS, there are currently thirty or more navigation satellites (hereinafter referred to as satellites) that orbit the earth on six semi-synchronous orbits (satellite orbits having revolution periods of half a sidereal day) having periods of approximately 12 hours, but in order to achieve constant positioning and time synchronization, it is necessary to choose an environment in which at least four satellites can always be captured.
Conventionally, the environmental conditions when installing navigation satellite antennas (hereinafter referred to as satellite antennas) have, for example, included conditions such as (a) that an open space of at least a certain elevation angle with respect to the horizontal plane is ensured at the satellite antenna installation position, (b) that there are no obstacles on the south side of the satellite antenna, (c) that there are no obstacles that reflect the radio waves of satellite signals near the satellite antenna, and (d) that there are no wireless communication devices in the vicinity of the satellite antenna that output wireless signals near the frequencies of the radio waves of the satellite signals. Satellite antenna installation positions were determined in accordance with these conditions, primarily on the basis of human confirmation work.
Furthermore, since the celestial positions of satellites change over time, it was necessary to provisionally install a satellite antenna on the basis of the above-mentioned environmental conditions, and then to monitor the satellite signal reception characteristics for a certain period of time (normally about one day), in order to confirm that the necessary number of satellite signals can constantly be captured.
If, as a result of monitoring the satellite signal reception characteristics in this way, the reception characteristics are poor, the satellite antenna installation position must then be determined once again, and such satellite antenna installation work procedures that are based on trial-and-error methodologies can not only lead to work delays, but can also cause reduced work efficiency and increased work costs. For this reason, reductions in the time required for optimal position determination and improvements in work efficiency in the installation of satellite antennas have been sought.
As a solution therefor, there is a method of estimating the satellite signal reception characteristics at the coordinates of a planned installation position by means of a simulation carried out beforehand, when installing a satellite antenna. As conventional art relating to methods for simulating satellite signal reception characteristics, systems in which the satellite signal reception characteristics are estimated by considering the influence of structures that can be obstacles to the reception of satellite signals have been proposed (see Non-Patent Documents 1 and 2).
These systems involve performing an analysis of the reception characteristics of satellite signals based on the following three models.
(1) Satellite orbit model: Estimating the position of a satellite by calculating the satellite orbit based on a Kepler orbit model using publicly available Keplerian orbital elements of the satellite.
(2) Signal propagation model: Estimating a radio wave propagation model of the satellite signals by considering the influence of direct waves, diffracted waves, and reflected waves of the radio waves from a satellite.
(3) Three-dimensional map model: Estimating, from three-dimensional map data, the reflection paths of satellite signals from non-line-of-sight (NLOS) satellites that cannot be directly viewed, and estimating a pseudorange error value.
From these models, it is considered to be possible to estimate the number of line-of-sight (LOS) satellites that can be directly viewed from any geographical point on a map, the area in which satellite positioning can be used, and the PDOP (position dilution of precision) value, which is position error information for the positioning.